Haptic Device With Controlled Traction Forces

ABSTRACT

A haptic device includes a substrate that is subjected to lateral motion such as lateral oscillation with one or more degrees of freedom together with modulation of a friction reducing oscillation in a manner that can create a shear force on the user&#39;s finger or on an object on the device.

This application is a continuation-in-part of copending U.S. applicationSer. No. 11/726,391 filed Mar. 21, 2007, and claims benefits andpriority of U.S. provisional application No. 61/196,660 filed Oct. 20,2008, wherein the entire disclosures of both applications areincorporated herein by reference.

CONTRACTUAL ORIGIN OF THE INVENTION

This invention was made with government support under Grant No.IIS-0413204 awarded by the National Science Foundation. The Governmenthas certain rights in the invention.

FIELD OF THE INVENTION

The present invention relates to a haptic device that can provide ashear force on a user's finger or an object on the surface of thedevice.

BACKGROUND OF THE INVENTION

Copending application Ser. No. 11/726,391 filed Mar. 21, 2007, of commonassignee discloses a haptic device having a tactile interface based onmodulating the surface friction of a substrate, such as glass plate,using ultrasonic vibrations. The device can provide indirect hapticfeedback and virtual texture sensations to a user by modulation of thesurface friction in response to one or more sensed parameters and/or inresponse to time (i.e. independent of finger position). A user activelyexploring the surface of the device can experience the haptic illusionof textures and surface features.

This haptic device is resistive in that it can only vary the forcesresisting finger motion on the interface surface, but it cannot, forinstance, re-direct finger motion.

It would be desirable to provide the variable friction benefits of thishaptic device and also to provide shear forces to a user's finger or anobject on the interface surface of the glass plate substrate.

SUMMARY OF THE INVENTION

The present invention provides a haptic device capable of providing aforce on a finger or object in contact with a substrate surface bysubjecting a substrate to lateral motion or lateral oscillation andmodulation of a friction reducing ultrasonic oscillation in a manner togenerate force. An embodiment of the present invention provides a hapticdevice comprising a substrate, one or more actuators for subjecting thesubstrate to lateral motion or lateral oscillation, and one or moreother actuators for subjecting the substrate to friction reducingultrasonic oscillation. A control device is provided for controlling theactuators in a manner to subject the substrate to lateral motion oroscillation and modulation of the friction reducing oscillation tocreate a force on the user's finger or on an object in contact with thesubstrate. Changing of the force in response to position of the user'sfinger or object on the substrate surface can provide a force field inthe plane of the substrate surface.

In an illustrative embodiment of the invention, a planar (flat-panel)haptic device modulates friction to provide the variable friction(friction reducing) effect by using vertical ultrasonic vibrations of ahorizontal substrate, such as a glass plate. The device also oscillatesthe substrate laterally in a horizontal plane with one degree of freedom(oscillation on one axis), two degrees of freedom (oscillation on twoaxes) or more while alternating between the low and high variablefriction states to create a non-zero net time-averaged shear force onthe user's finger or on an object in contact with the substrate. Forexample, for one degree of freedom of lateral oscillation, as thesubstrate moves in one direction in a horizontal plane, the friction isreduced (low friction state). As the substrate moves in the oppositedirection, the friction is increased (high friction state). The nettime-averaged force on the user's finger or on a part is non-zero andcan be used as a source of linear shear force applied to a finger or toan object in contact with the surface.

For two degrees of freedom lateral oscillation (e.g. on x and y axes),the substrate may be moved in a swirling manner to provide circular,in-plane motion (in the plane of the substrate surface). As thesubstrate swirls, its velocity vector will at one instant line up withthe desired force direction. Around that instant, the substrate is setto its high friction state and an impulse of force is thereby applied tothe user's finger or to an object. During the remainder of the “swirl”cycle, the substrate is set to the low friction state so that itnegligibly effects the force on the finger or object. Since the velocityvector passes through all 360° during the swirl, forces can be createdin any in-plane direction.

Alternatively, in another embodiment, the substrate may be oscillated ina single direction in the horizontal plane, but this single directionmay be changed to match the desired force direction at any instant. Instill another embodiment of the invention, the substrate may beoscillated on three axes (x and y translations and an in-plane rotationabout a vertical axis). It should further be understood that the lateraloscillations need not be sinusoidal, need not be of uniform amplitude,and need not continue indefinitely. For instance in another embodimentof the invention, a single lateral motion or a short series of lateralmotions or displacements of the substrate may be used.

The present invention is advantageous to provide a haptic device thatprovides guiding forces to a user's fingers to enable the user toexplore a display. Even an active propulsion of the user's finger may beof use to provide a compelling haptic experience. The present inventionalso is advantageous to provide a haptic device that provides guidingforces to one or more objects on the substrate in a manner to provideobject or parts manipulation device for use in parts feeding, in roboticapplications, and in manufacturing applications.

Advantages of the present invention will become more readily apparentfrom the following detailed description taken with the followingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a perspective view of a haptic device TPaD capable ofvariable friction effect. FIG. 1B is a perspective view of a mount forthe haptic device TPaD.

FIG. 2 is a perspective view of the haptic device TPaD adhered in themount.

FIG. 3 is a schematic perspective view of a planar haptic deviceincluding the haptic device TPaD and other components pursuant to theinvention. FIG. 3A is a view of a force measurement device used formeasuring lateral force. FIG. 3B is a view of an alternative forcemeasurement device used for measuring lateral force.

FIG. 4 is a schematic view of a control system for controlling theactuators in a manner to subject the substrate to lateral oscillation insynchrony with the friction reducing oscillation to create a shear forceon the user's finger or an object in contact with the substrate.

FIG. 5 is a schematic view of a finger position sensor system for use inpracticing an embodiment of the invention.

FIG. 6A is a schematic view showing rightward movement of the TPaD withhigh friction to create a rightward impulse on the finger. FIG. 6B is aschematic view showing leftward movement of the TPaD with low frictionto prepare for a another rightward impulse.

FIG. 7 shows force impulses in unfiltered force signals where TPaD turnson (low friction state) at Φ_(on)=40° and remains in the on state for180°.

FIG. 8 shows net force changes as Φ_(on) is rotated over time where theunfiltered 40 Hz force signal has the same frequency as the lateralmotion of the TPaD. The Φ_(on) is rotated through all phase angles at0.5 Hz and the net (filtered) force changes accordingly. The circledmaximum force points occur at the “optimum Φ_(on)” values.

FIG. 9 shows a plot of the force versus Φ_(on) wherein as Φ_(on) ischanged, the net force shifts from leftward to rightward and back again.The optimum Φ_(on) values that produce the maximum leftward andrightward forces are marked. Additionally, one of the two Φ_(on) valuesthat produces zero net force is marked.

FIG. 10 is a plot of maximum force versus RMS (root mean square)displacement where the relationship between amplitude of oscillation andmaximum net force for various lateral oscillation frequencies whereF_(N)=392 mN, μ_(glass)=0.70, and μ_(on)=0.06.

FIG. 11 is a replot of the data of FIG. 10 as a function of TPaD RMSvelocity.

FIG. 12 illustrates a line-source force field and the Φ_(on) requestthat the SHD controller uses to generate the line-source field.

FIG. 13 shows the data, net force versus finger position on the TPaD,from four different force fields comprising two line source force fieldsand two line-sink force fields.

FIG. 14A shows plots of potential and force versus x axis position. FIG.14B is a plan view of 2 line-sinks (force fields) and 1 line-source(force field). FIG. 14C shows schematically a haptic toggle switcheffect.

FIG. 15 is a schematic view of a two degree-of-freedom haptic devicewhere the haptic device TPaD is mounted on a compliant support.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a haptic device referred to hereafter asa surface haptic device (SHD) capable of providing a force on a fingeror object in contact with a haptic substrate surface by subjecting thesubstrate to lateral motion or lateral oscillation and modulation of afriction reducing oscillation. An embodiment of the present inventionprovides a haptic device comprising a substrate such as a flat glass orother plate, one or more actuators for subjecting the substrate tolateral motion or lateral oscillation, and one or more other actuatorsfor subjecting the substrate to friction reducing ultrasonicoscillation. The actuators are controlled in an embodiment by a computercontrol device to subject the substrate to lateral motion or lateraloscillation in synchrony with modulation of the friction reducingoscillation in a manner to create a shear force on the user's finger oran object in contact with the substrate surface. The present inventionenvisions subjecting the substrate to lateral motion or oscillation on asingle axis (e.g. X axis) or on multiple (e.g. X and Y axes) axes asdescribed below.

In an illustrative embodiment, the present invention can be practicedusing a variable friction haptic device TPaD (“Tactile Pattern Display”)of the illustrative type shown in FIGS. 1A, 1B and 2 having a substrate100 with a working haptic surface and one or more actuators (vibrators)operably associated with the substrate in a manner to impart vibration(oscillation) thereto in a manner to provide a variable frictioncapability as described in copending U.S. application Ser. No.11/726,391 filed Mar. 21, 2007, and copending U.S. application Ser. No.12/383,120 filed Mar. 19, 2009, of common assignee, the disclosures ofwhich is incorporated herein by reference. The variable friction hapticdevice VFHD of the copending application is referred to below as thevariable friction haptic device TPaD.

Referring to FIGS. 1A, 1B and 2, a variable friction haptic device TPaDaccording to an illustrative embodiment of the invention is shown havinga substrate 100 comprising a piezoelectric bending element 102 in theform of piezoelectric sheet or layer member attached to a passivesubstrate sheet or layer member 104 with a touch (haptic) surface 104 ato provide a relatively thin laminate structure and thus a slim hapticdevice design that can provide advantages of slimness, high surfacefriction, inaudiblity and controllable friction. A relatively thinhaptic device can be made of a piezo-ceramic sheet or layer glued orotherwise attached to a passive support sheet or layer 104. When voltageis applied across the piezoelectric sheet or layer 102, it attempts toexpand or contract, but due to its bond with the passive support sheetor layer 104, cannot. The laminate will have a curved shape with asingle peak or valley in the center of the disk when the piezoelectricsheet or layer 102 is energized. The resulting stresses cause bending.The greater the voltage applied to the piezoelectric sheet or layer, thelarger the deflection. When the piezoelectric bending element is excitedby a positive excitation voltage, it bends with upward/positivecurvature. When the piezoelectric bending element is excited by anegative excitation voltage, it bends with a downward/negativecurvature. When sinewave (sinusoidal) excitation voltage is applied, thepiezoelectric bending element will alternately bend between thesecurvatures. When the sinewave excitation voltage is matched in frequencyto the resonant frequency of the substrate 100, the amplitude ofoscillation is maximized. A mount 150 may be used to confine the bendingto only one desired mode or to any number of desired modes. It ispreferred that all mechanical parts of the haptic device vibrate outsideof the audible range. To this end, the substrate 100 preferably isdesigned to oscillate at resonance above 20 kHz.

For purposes of illustration and not limitation, a thickness of thepiezoelectric member 102 can be about 0.01 inch to about 0.125 inch. Anillustrative thickness of the substrate member 104 can be about 0.01 toabout 0.125 inch. The aggregate thickness of the haptic device thus canbe controlled so as not exceed about 0.25 inch in an illustrativeembodiment of the invention.

As shown in FIGS. 1A, 1B and 2, the disk-shaped haptic device isdisposed in a mount 150 in order to confine the vibrations of thebending element disk to the 01 mode where the 01 mode means that thelaminate has a curvature with a single peak or valley in the center ofthe disk when the piezoelectric sheet or layer is excited. The mount 150can be attached to the piezoelectric disk along a thin ring or annularsurface 150 a whose diameter can be ⅔ of the diameter of thepiezoelectric disk. The same very low viscosity epoxy adhesive can beused for the bond to the mount 150 as used to bond the piezoelectricdisk and the glass substrate disk. The inner height of the mount 150 issomewhat arbitrary and can also be made as thin as a few millimeters.The mount 150 is adapted to be mounted on or in an end-use product suchas including, but not limited to, on or in a surface of an motor vehicleconsole, dashboard, steering wheel, door, computer, and other end-useapplications/products.

A transparent haptic device preferably is provided when the hapticdevice is disposed on a touchscreen, on a visual display, or on aninterior or exterior surface of a motor vehicle where the presence ofthe haptic device is to be disguised to blend with a surrounding surfaceso as not be readily seen by the casual observer. To this end, either orboth of the piezoelectric member 102 and the substrate member 104 may bemade of transparent material. The piezoelectric element 102 includesrespective transparent electrodes (not shown) on opposite sides thereoffor energizing the piezoelectric member 102.

For purposes of illustration and not limitation, the substrate 104 maybe glass or other transparent material. For the electrode material, thinfilms of the In₂O₃—SnO₂ indium tin oxide system may be used as describedin Kumade et al., U.S. Pat. No. 4,352,961 to provide transparentelectrodes. It is not necessary to employ transparent piezoelectricmaterial in order to achieve a transparent haptic device. It will beappreciated that passive substrate sheet 104 may be made of atransparent material such as glass, and that it may be significantlylarger in surface area than piezoelectric sheet 102. Piezoelectric sheet102 may occupy only a small area at the periphery of passive substratesheet 104, enabling the rest of passive substrate sheet 104 to be placedover a graphical display without obscuring the display. Thepiezoelectric material can include, but is not limited to, PZT (Pb(Zr,Ti)O₃)-based ceramics such as lanthanum-doped zirconium titanate (PLZT),(PbBa)(Zr, Ti)O₃, (PbSr)(ZrTi)O₃ and (PbCa)(ZrTi)O₃, barium titanate,quartz, or an organic material such as polyvinylidene fluoride.

Those skilled in the art will appreciate that the invention is notlimited to transparent piezoelectric and substrate members and can bepracticed using translucent or opaque ones, which can be colored asdesired for a given service application where a colored haptic device isdesired for cosmetic, security, or safety reasons. Non-transparentmaterials that can be used to fabricate the substrate member 104include, but are not limited to, steel, aluminum, brass, acrylic,polycarbonate, and aluminum oxide, as well as other metals, plastics andceramics.

Design of a circular disk-shaped haptic device TPaD will includechoosing an appropriate disk radius, piezo-ceramic disk thickness, andsubstrate disk material and thickness. The particular selection madewill determine the resonant frequency of the device. A preferredembodiment of a disk-shaped haptic device employs a substrate diskhaving a thickness in the range of 0.5 mm to 2 mm and made of glass,rather than steel or other metal, to give an increase in resonantfrequency (insuring operation outside the audible range) withoutsignificantly sacrificing relative amplitude.

Those skilled in the art will appreciate that the design of thepiezoelectric bending element 102 and substrate 104 are not constrainedto the circular disk shape described. Other shapes, such as rectangularor other polygonal shapes can used for these components as will bedescribed below and will exhibit a different relative amplitude andresonant frequency.

With respect to the illustrative disk-shaped haptic device TPaD of FIGS.1A, 1B and 2, the amount of friction felt by the user on the touch(haptic) surface 104 a of the haptic device is a function of theamplitude of the excitation voltage at the piezoelectric member 102. Theexcitation voltage is controlled as described in the Example below andalso in copending application Ser. No. 11/726,391 filed Mar. 21, 2007,and copending application Ser. No. 12/383,120 filed Mar. 19, 2009, bothof which are incorporated herein by reference. The excitation voltage isan amplitude-modulated periodic waveform preferably with a frequency ofoscillation substantially equal to a resonant frequency of the hapticdevice. The control system can be used with pantograph/optical encodersor with the optical planar (two dimensional) positioning sensing systemor with any other single-axis or with two-axis finger position sensorswhich are described in copending application Ser. No. 11/726,391incorporated herein by reference, or with any other kind of fingerposition sensor, many of which are known in the art.

EXAMPLES One Degree of Freedom Planar Haptic Device

Referring to FIG. 3, an illustrative planar surface haptic device SHDpursuant to an illustrative embodiment of the invention is shownincorporating the disk-shaped haptic device TPaD of FIGS. 1A, 1B and 2hereafter referred to as TPaD. The disk-shaped haptic device TPaD wasconstructed using a single circular disk of piezoelectric bendingelement (Mono-morph Type) and a single circular disk of glass platesubstrate to generate the ultrasonic frequency and amplitude necessaryto achieve the indirect haptic effect of friction reduction. Thepiezoelectric bending element disk comprised PIC 151 piezo-ceramicmaterial (manufactured by PI Ceramic, GmbH) having a thickness of one(1) millimeter and diameter of 25 millimeters (mm). The glass platesubstrate disk comprised a thickness of 1.57 mm and a diameter of 25 mm.The piezo-ceramic disk was bonded to the glass substrate disk using avery low viscosity epoxy adhesive such as Loctite E-30CL Hysol epoxyadhesive. The disk-shaped haptic device was disposed in a mount made ofaluminum and attached to the piezoelectric disk along a thin ring orannular surface 150 a whose diameter was ⅔ of the diameter of thepiezoelectric disk. The same very low viscosity epoxy adhesive was usedfor the bond to the mount 150 as was used to bond the piezoelectric diskand the glass substrate disk.

The haptic device SHD further includes a linear actuator 200, such as avoice coil, connected by coupling rod 211 to a linear slider 210 onwhich the haptic device TPaD fixedly resides for movement therewith. TheTPaD can be held in fixed position on the slider 210 by any connectionmeans such as a clamp, glue, screws, or rivets. The linear slider 210 ismovably disposed on support 212 on a fixed base B for movement on asingle X axis. A linear voice coil actuator 200 is sinusoidallyactivated at frequencies between 20 and 1000 Hz, causing the slider 210and haptic device TPaD thereon to move oscillate laterally in theX-direction at the same frequency. When voice coil actuator 200 issinusoidally activated at the resonant frequency of this system, theamplitude of lateral oscillations is increased although the invention isnot limited to such sinusoidal activation.

Friction is modulated on the glass plate substrate surface 104 a of thehaptic device TPaD by applying a 39 kHz sinusoid to the piezoelectricelement 102 mounted on the underside of the glass plate substrate 104.The 39 kHz signal is generated by a AD9833 waveform generator chip andamplified to +0-20V using an audio amplifier. When applied to thepiezoelectric element 102, it causes resonant vibrations of the glassplate substrate. These vibrations produce a squeeze film of airunderneath the fingertip, leading to a reduction of friction. At highexcitation voltages, the friction between the glass plate substrate anda finger is approximately μ=0.15, while at zero voltage, the surface hasthe friction of normal glass (approximately μ=0.95).

A programmable integrated circuit (PIC-18F4520) generates the lowfrequency signal for the voice coil (x-actuator) and issues the commandto the wave form signal generator (AD98330), FIG. 4, to start/stop the39 kHz signal of the piezoelectric element 102. Since it provides bothfunctions, it can dictate the phase relationship between the frictionlevel of the haptic device TPaD and the lateral motion. A control systemhaving a microcontroller with the PIC or other controller and fingerposition sensor 250 is shown in FIG. 4. FIG. 4 shows an X axis-actuatorto oscillate the linear slider 210 on the X-axis and also a Yaxis-actuator for use with a two degree-of-freedom planar haptic devicedescribed below where the TPaD is oscillated on the X-axis and Y-axisconcurrently.

To measure finger position, a single axis of the two-axis fingerpositioning system 250 can be used. This system is of a type similar tothe two-axis finger position sensors which are described in copendingapplication Ser. No. 11/726,391, however the infrared light emittingdiodes of that system have been replaced with laser line generators 252and Fresnel lenses 254 which produce a collimated sheet of lightstriking linear photo diode array 256, FIG. 5. The collimated sheet oflight is placed immediately above the surface 104 a of the TPaD and afinger touching the TPaD surface 104 a interrupts that sheet of light,casting a shadow on linear photo diode array 256. A PIC microcontrollerreads the output of the linear photo diode array 256 and computes thecentroid of the finger's shadow, which is used as a measure of fingerposition.

Characterization of Force Generation

In the one degree-of-freedom embodiment, forces are created byalternating between low and high friction states at the same frequencythat the haptic device TPaD is being oscillated laterally in-plane. Toproduce a net leftward force, the haptic device TPaD s set to highfriction while its velocity is leftward and set to low friction when itsvelocity is rightward. The haptic device TPaD alternates between pushingthe user's finger to the left and slipping underneath the finger back tothe right. This “pushslip” cycle repeats itself, and the series ofstrong leftward impulses followed by weak rightward impulses results ina net force to the left. These impulses can be seen in the unfilteredforce signal in FIG. 8, the lateral or shear force between the surface104 a of the SHD and the fingertip being measured using a onedegree-of-freedom tension/compression load cell operably connected to aproxy fingertip as shown in FIG. 3A. The load cell can move verticallyon a linear slider. The weight of the load cell and the fingertip issuspended by a low-stiffness spring. Vertical position of the fingertipis adjustable via a thumb nut. After the fingertip is lowered to withinclose proximity (less than 0.5 mm) of the TPaD, the normal force iscontrolled by adding weight to the L-shaped finger as shown. A normalload of 392 mN (40 g) was used for this Example. This arrangement allowsforce on the proxy finger, including the effects of lateraloscillations, to be measured with great accuracy. The proxy fingertipused in this Example comprised a grape wrapped in sandpapered electricaltape as the proxy finger pad. The proxy fingertip was secured to theL-shaped aluminum “finger” shown in FIG. 3A with electrical tape, andthe aluminum finger was threaded onto the load cell. There is somecompliance in the fingertip-to-finger connection, but since there issimilar compliance in the human finger, this is appropriate.

Alternatively, the lateral or shear force between the surface 104 a ofthe SHD and the fingertip also can be measured by mounting the base B ona support assembly that allows the entire assembly to move laterallywith essentially no friction, FIG. 3B. For example, the base B can bemounted on a heavy brass mass or plate 230, which rests on soundinsulating foam plate 232. The sound insulating foam plate 232 in turnrests on precision ground steel plate 234, which rests on three largesteel balls 236. The steel balls in turn rest on a lowermost precisionground steel plate B2 that serves as the base of the measuring system.The brass mass or plate 230 is connected to load cell 240 via alow-stiffness spring 238 so that the only component restricting lateralmotion is the load cell and so that all lateral forces acting on thefinger must be matched by forces on the load cell. The combination ofthe mass of the brass mass or plate 230 and the low stiffness of spring238 acts like a mechanical low pass filter so that lateral oscillationshave minimal effect on the measurement of the load cell. As aconsequence, the output of the load cell is an accurate measure of theaverage force on the fingertip.

FIGS. 6A, 6B illustrate a similar “push slip” cycle to generate theopposite net force to the right wherein strong rightward impulses arefollowed by weak leftward impulses resulting in a net force to the righton a user's finger.

The Effect of Phasing on Force:

By changing the phase angle between the lateral velocity and the hapticdevice TPaD on/off signal, the direction and magnitude of the net forcecan be changed. For explanation, the term Φ_(on) is defined as the phaseangle of the lateral velocity when the haptic device TPaD turns on (lowfriction state on). This concept is shown graphically in FIG. 7. In allof the data presented, the TPaD is in the on state for half (180°) ofthe full cycle of lateral oscillation.

To determine which phasing creates the largest magnitude force, Φ_(on)was rotated slowly from 0 to 360° over the course of about 2 seconds. Tofind the net force, the unfiltered force data was passed through asecond-order, lowpass, butterworth, zero-phase filter (f_(cutoff)=10Hz). The filtered force signal is shown in FIG. 8. The circled maximumforce points correspond to the two “optimum Φ_(on)” values for thisparticular frequency and amplitude of lateral oscillation. These maximumnet forces in the 100 mN range are easily perceivable to the applicants.Moreover, literature shows that this magnitude is generally perceivableto humans; see references [5] [1] [8].

FIG. 8 shows that the net force changes as Φ_(on) is rotated over time.The Φ_(on) value in any given velocity cycle was found by comparing theTPaD status signal to the velocity signal in a manner similar to thecomparison in FIG. 7. That data was then plotted against the filteredforce data in FIG. 8. The result in FIG. 9 provides more specificinformation about the values of Φ_(on) that optimize force. Although theexact relationship between force and Φ_(on) is dependent on oscillationamplitude and frequency, this data is representative of a wide range ofamplitudes that produce forces noticeable to a human. With no delays inthe system, one would expect that turning the TPaD on at 0° wouldproduce the largest leftward force and Φ_(on)=180° would produce thegreatest rightward force. The data shows that the optimum angles areinstead Φ_(on)=340°, and Φ_(on)=160°, respectively. The need for thisphase advance may be due to the time required to create and decay thesqueeze film, although applicants do not intend or wish of be bound byany theory in this regard. Also note that zero net force is expected atΦ_(on)=270° but occurs at Φ_(on)=250°.

One skilled in the art will recognize that force can be controlled notjust by phasing, but also by modulating the amount of time that the TPaDsubstrate is in the relatively high friction state. Force may be reducedby reducing the amount of a cycle for which friction is high.

The Effect of Oscillation Amplitude on Force:

It was found experimentally that as the amplitude of lateraldisplacement increases, the average net force increases proportionallyat first and then saturates. FIG. 10 shows this trend for variouslateral oscillation frequencies. Each data point represents the netforce produced by a particular amplitude of oscillation at optimumΦ_(on).

The asymptotic behavior in FIG. 10 is due to the nature of coulombfriction. Once amplitudes are high enough to keep the finger and TPaDpredominately in sliding contact, the finger will experience a force ofμ_(glass) F_(N) when the velocity is in one direction and −μ_(on) F_(N)when velocity is in the other direction. Here, μ_(glass) is the kineticcoefficient of friction of the glass; μ_(on) is the kinetic coefficientof friction of the TPaD when it is in its lowest friction state; andF_(N) is the normal force.

To find the theoretical maximum net force that the SHD can create, weassume that the finger experiences each of the two force levels for halfof the total cycle. The time-averaged force is then just the simpleaverage of the two force levels. Therefore, the equation for the maximumnet force, F., is

F _(max)=[(μ_(glass)−μ_(on))F _(N)]/2   Eqn (1)

The value of the asymptote line in FIG. 10 is calculated using Eqn 1,where μ_(glass)=0.70 was found by recording the maximum force whilesweeping the proxy finger across the surface while the TPaD and lateraloscillator are quiescent; μ_(on)=0.06 was found similarly but with theTPaD turned on; and F_(N)=392 mN was from 40 g of weight.

Frequency Selection:

It is important to note that since the force is applied in impulses atfrequencies between 20 and 1000 Hz, the user is aware of not only theoverall force in one direction, but also the undesirable underlyingvibration of the TPaD. It is well known in the field of psychophysicsthat the human fingertip is sensitive to vibrations in the range of 20Hz to about 500 Hz, with a peak in sensitivity at about 250 Hz. We havefound the best performance of the SHD to be either at high frequencies(e.g., 850 Hz) where the lateral vibrations are not very noticeable, orat about 40 Hz where the vibrations are noticeable but not unpleasant.

A design method for reducing the finger's exposure to the lateralvibration is to keep the TPaD continually turned on (low friction stateon) until a force production is needed. In this strategy the squeezefilm isolates the user from the underlying low frequency vibrationmaking it almost unnoticeable until force is applied. Another method isto turn off the lateral vibrations except when they are needed.

The Effect of Finger Exploration Velocity:

When the amplitude of oscillation is large enough to bring the forcesnear F_(max.) increasing amplitude further provides negligible increaseto the force on a stationary finger. On the other hand, if the user isactively exploring the surface, their finger velocity could cause therelative velocity between the finger and plate to become small, reducingthe net force. Therefore, the higher the finger exploration velocities,the higher the oscillation amplitude required to maintain the targetforce.

An idea for acceptable finger exploration velocities can be gained byplotting the same data in FIG. 10 against velocity instead ofdisplacement. In FIG. 11, it can be seen that a TPaD oscillating at 77Hz reaches about 85% of maximum force production around 20 mm/s RMSvelocity. In FIG. 11, the force produced by 77 Hz oscillation is seen tobe very sensitive to changes in RMS velocity below 20 mm/s, butinsensitive above 20 mm/s. Therefore, it is believed that the forcesproduced by a SHD oscillating at 20 mm/s are susceptible to fingervelocity changes, but a SHD running at 60 mm/s will allow the finger totravel at speeds up to about 40 mm/s before a significant reduction inforce occurs.

Displaying Force Fields:

Since the SHD is effectively a source of force, it is possible to createor display any arbitrary force field. One could chose to display aspring, damper, or other primitive, but for the sake of example we willdescribe the display of line sinks and sources. At any given moment intime, the device has a constant force field across its surface, so tocreate the perception of a spatially varying force field, it isnecessary to change force as a function of finger position. In practice,as the finger moves across the surface, Φ_(on) is adjusted to producethe force of desired direction and magnitude. In FIG. 12, a top-view isprovided of what a line-source force field looks like, and the Φ_(on)command used to generate such a field.

Note the similarities between the commanded Φ_(on) in FIG. 12 and theForce versus Φ_(on) relationship in FIG. 9. To create a line source, onewants zero force along the centerline, so Φ_(on)=250° at x=0. On theright and left edges of the vector field where maximum force isrequired, the Φ_(on) command takes on the optimum Φ_(on) angles of 340°and 160°.

Force Fields:

FIG. 13 shows the data from four different force fields. There are twoline-sources and two line-sinks, each of which has a “stiff and“compliant” version. The raw data has been provided in the force vs.position format, but to provide a more intuitive idea for the tactileexperience, applicants have also integrated the data to form the“potential function”. The potential function is defined asV(x)=¶,F(x)dx, where F(x) is the force on the finger as a function ofposition, x. The results from Robles-De-La-Torre and Hayward references[6] [5] suggest that the shape of the potential function is similar tothe perceived shape of a virtual bump or hole.

When viewing the data from the perspective of the potential function,instead of seeing a stiff planar line-source, one sees a steep bump inthe surface. Similarly, the compliant planar line-sink can be thought ofas a shallow hole in the surface.

It is possible to build on the potential function concept to create moresophisticated haptic behaviors, such as the toggle switch illustrated inFIGS. 14A, 14B, and 14C. The haptic toggle switch is two low potentialregions (sinks) separated by a high potential region (source), FIGS.14A, 14B. When sliding the finger from one low potential region towardthe next, the finger tends to “pop” into the low potential region, muchlike flipping a physical toggle switch, FIG. 14C.

The illustrative planar (flat panel) haptic display SHD described aboveis capable of applying and controlling the net shear force on a finger.As with any controllable force source, it allows one to display forcefields of one's choosing when coupled with finger position feedback. Thecapability of displaying line sources and sinks has been demonstratedand they can be viewed as planar entities, or 3D protrusions anddepressions. It can be extrapolated that the SHD is a tool capable ofdisplaying planar springs, dampers, masses, and the illusions of surfacefeatures.

The illustrative haptic device SHD provides a planar haptic displaycapable of applying any arbitrary shear force to a finger. It would havethe capability of displaying a two dimensional (2D) world composed ofsprings, dampers, masses, and other forces, but also, by using the ideathat lateral force can create the illusion of shape, the SHD can producethe illusion of three dimensional (3D) textures and shape on its 2Dsurface.

Two Degree of Freedom Planar Haptic Device

A planar haptic device having two degrees of freedom of oscillation ofthe substrate can be constructed in view of the above description of theone degree-of- freedom illustrative haptic device SHD of FIG. 3.

For two degrees of freedom in-plane oscillation, the haptic device TPaDshown in FIG. 3 can be oscillated in-plane on multiple axes such on theX axis and the Y axis. For example, two degrees of freedom of motion areprovided by designing a compound slider on which the haptic device TPaDresides to slide on orthogonal X and Y axes, see X and Y axes on FIG. 3.The compound slider would have the capability to move the haptic deviceTPaD independently on the X axis and Y axis. For example, the compoundslider would have a first slider like slider 210 for X axis oscillationand a second slider mounted on or underneath the first slider forindependent Y axis oscillation. The first slider would be actuated tooscillate on the X axis by linear actuator 200 shown and the secondslider would be actuated to oscillate on the Y axis by a second linearactuator (not shown) in the form of a similar speaker having an actuatorrod connected to the Y-axis slider. A finger position sensor of the typeshown in FIG. 5 can be used to sense X and Y finger positions for inputto a control system that changes the phase angle Φ_(on) between lateralvelocities of the substrate and the TPaD on/off signal as desired togenerate shear forces.

This embodiment for two degrees of freedom thus involves having separateactuators for oscillating the TPaD on the X axis and Y axis.Electromagnetic actuators (such as voice coils), piezoelectric bendingactuators, shape memory alloy actuators, artificial muscle actuators(http://www.artificialmuscle.com/) and others are possible choices forthese actuators. In general, it will minimize the actuator effortrequired if the haptic device TPaD and its mount 150 are resonant forboth X and Y vibrations at the frequency of oscillation.

Oscillation of the haptic device TPaD on the X axis and Y axis may becontrolled to generate a swirling motion of the substrate in a manner tocreate a circular, in-plane motion (in the plane of the substratesurface 104 a). As the substrate swirls, its velocity vector will at oneinstant line up with the desired force direction. Around that instant,the substrate is set to its high friction state and an impulse of forceis applied to the user's finger or an object. During the remainder ofthe “swirl” cycle, the substrate is set to the low friction state sothat it negligibly affects the force on the finger or object. Since thevelocity vector passes through all 360° during the swirl, forces can becreated in any in-plane direction. In this embodiment of the invention,the ultrasonic vibrations normal to the substrate are combined with alower frequency, higher amplitude lateral vibration (i.e. motions in theplane of the surface to generate the “swirls”) as described.

As a consequence, each point on the glass plate surface of the hapticdevice TPaD will execute a small, circular, counterclockwise (lookingfrom above) motion in the X-Y plane to generate the swirling pattern ofmotion. The swirling motion is completely analogous to the X-directionoscillations generated above by linear actuator 200 in the sense thatthe same considerations of frequency and amplitude apply. However,because the motion now occurs along two axes of glass plate substrate,the effect of friction modulation is not the same. In particular, thenet force never goes to zero (or changes in magnitude), it simplychanges direction. Also, because the force is always in the samedirection as the velocity of the device, and that velocity is constantlychanging, the average force will not be as large as in the single axisembodiment. It can be shown, assuming friction dependence as above, thatthe average force has magnitude (μ_(on)N)/π and a direction of Φ.

The phase(s) of the swirling motion during which the ultrasonicvibration for friction reducing is switched on or off (or modulated) canbe varied under computer control to create edges or other hapticeffects. The modulation can be in response to measured finger position,or for some haptic effects a measurement of finger position is notnecessary.

Another embodiment of the present invention for a two degree-of-freedomhaptic device SHD is shown in FIG. 15 and involves mounting the hapticdevice TPaD on a compliant support such as a motor mount 410 supportedon flexible metal legs 400 so that the haptic device TPaD is free tomove around within some limits as needed. The flexible legs 400 areconnected at the bottom to a fixed base. The TPaD is fixedly mounted ona mounting plate 430 that is connected rigidly by corner posts 422 tothe motor mount 410. An eccentric mass motor MM (such as those used inpagers) can be mounted on the motor mount 410 and includes eccentricmass 411. The motor's output shaft axis is normal to the TPaD hapticsurface. The eccentric mass motor MM via its output shaft spins theeccentric mass to produce a rotating reaction force at the frequency ofrotation. This, in turn, will cause the motor mount 410 and the TPaDsubstrate 104 thereon to vibrate in a swirling pattern.

Embodiments of the invention described allow computer(software)-controlled haptic effects to be displayed on the glass platesubstrate surface, including not only variable friction but also lateralforces that actively push the finger or object across the surface.Stronger haptic effects are possible. An additional use is alsopossible, not as a haptic display but instead as a mechanism for drivingsmall objects around a surface under computer control, as might beuseful in parts feeding or similar applications in robotics ormanufacturing.

In the described embodiments, the haptic device TPaD is ultrasonciallyvibrated for the friction reduction effect as one unit. As analternative embodiment, more than one ultrasonic actuator can be used sothat different areas of the glass plate surface have differentultrasonic amplitudes, perhaps each modulated to correspond to differentphases of the swirly motion. Another way to attain spatial variation ofultrasonic amplitude across the glass plate surface, is to make use ofthe nodal patterns of ultrasonic vibration (see copending applicationSer. No. 12/383,120 filed Mar. 19, 2009, or to combine this with morethan one ultrasonic frequency, or with ultrasonic actuators driven withdifferent phases.

It should be appreciated that the present invention is not limited toplanar substrate surfaces. For example, the traction forces could begenerated at the surface of a cylindrical knob by creating ultrasonicvibrations in the radial direction, and “lateral” oscillations in theaxial and/or circumferential directions. Indeed, any surface will have asurface normal and two axes that lie in the surface, at least locally.Ultrasonic vibration along the normal and lower frequency vibrationalong one or two in-surface axes can be coordinated to generate tractionforces.

There is no reason that the lateral oscillations need to be persistent.In many applications, it is necessary to apply active traction forcesfor brief instants only. In such cases, the lateral oscillations can beturned off until they are needed to generate the traction force. Indeedfor some haptic effects only a single cycle or even only a half-cycle ofa lateral oscillation may suffice. The amplitude or number of lateraloscillations may be selected to be sufficient to move the user's fingera desired distance, or to apply a force to it for a desired duration,and then the lateral oscillations may be discontinued.

Although the invention as been described with respect to certainillustrative embodiments thereof, those skilled in the art willappreciate that changes and modifications can be made thereto within thescope of the invention as set forth in the pending claims.

REFERENCES

[1] M. Biet, F. Giraud, and B. Lemaire-Semail. Implementation of tactilefeedback by modifying the perceived friction. European Physical JournalAppl. Phys., 43:123135, 2008.

[2] S. M. Biggs, S. Haptic Interfaces, chapter 5, pages 93-115.Published by Lawrence Erlbaum Associates, 2002.

[3] M. Minsky. Computational Haptics: The Sandpaper System forSynthesizing texture for a force-feedback display. PhD thesis,Massachusetts Institute of Technology, Cambridge, Mass., 1995.

[4] J. Pasquero and V. Hayward. Stress: A practical tactile display withone millimeter spatial resolution and 700 hz refresh rate. Dublin,Ireland, July 2003.

[5] G. Robles-De-La-Torre. Comparing the Role of Lateral Force DuringActive and Passive Touch: Lateral Force and its Correlates areInherently Ambiguous Cues for Shape Perception under Passive TouchConditions. pages 159-164, 2002.

[6] G. Robles-De-La-Torre and V. Hayward. Force can overcome objectgeometry in the perception of shape through active touch. Nature,412:445-448, July 2001.

[7] M. Takasaki, H. Kotani, T. Mizuno, and T. Nara. Transparent surfaceacoustic wave tactile display. Intelligent Robots and Systems, 2005.(IROS 2005). 2005 IEEE/RSI International Conference on, pages 3354-3359,Aug. 2005.

[8] V. Vincent Levesque and V. Hayward. Experimental evidence of lateralskin strain during tactile exploration. In Proc. of Eurohaptics, Dublin,Ireland, July 2003.

[9] T. Watanabe and S. Fukui. A method for controlling tactile sensationof surface roughness using ultrasonic vibration. Robotics andAutomation. 1995. Proceedings., 1995 IEEE International Conference On,1:1134-1139 vol. 1, May 1995.

[10] L. Winfield, J. Glassmire, J. E. Colgate, and M. Peshkin. T-pad:Tactile pattern display through variable friction reduction. WorldHaptics Conference, pages 421-426, 2007.

[11] A. Yamamoto, T. Ishii, and T. Higuchi. Electrostatic tactiledisplay for presenting surface roughness sensation. pages 680-684,December 2003.

1-29. (canceled)
 30. A haptic device comprising a substrate having asurface, one or more actuators that move the substrate and generate anet shear force that is applied to a user's finger touching thesubstrate surface, a finger position sensor that detects and measureslocation of the user's finger relative to the substrate surface, and acontrol device that controls the one or more actuators and varies themagnitude and direction of the net shear force applied to the user'sfinger as a function of at least the measured location of the user'sfinger, wherein a force field is created that acts upon the user'sfinger.
 31. The haptic device of claim 30 in which the integral of theforce field is a potential function.
 32. The haptic device of claim 31in which the potential function contains one or more sources.
 33. Thehaptic device of claim 31 in which the potential function contains oneor more sinks.
 34. The haptic device of claim 31 in which the potentialfunction contains both sources and sinks.
 35. The haptic device of claim31 in which the potential function is that of a toggle switch.
 36. Thehaptic device of claim 30 in which the force field resembles a spring.37. The haptic device of claim 30 in which the force field resembles adamper.
 38. The haptic device of claim 30 wherein the substratecomprises a flat plate.
 39. The haptic device of claim 30 wherein thenet shear force acts along a single axis.
 40. A method of generating aforce field that acts upon a user's finger that touches a surface of asubstrate of a haptic device, comprising detecting and measuring alocation of the user's finger relative to the substrate surface,controlling one or more actuators that move the substrate and generate anet shear force that is applied to a user's finger touching thesubstrate surface so as to vary the magnitude and direction of the netshear force applied to the user's finger as a function of at least themeasured location of the user's finger.
 41. The method of claim 40 inwhich the integral of the force field is a potential function.
 42. Themethod of claim 41 in which the potential function contains one or moresources.
 43. The method of claim 41 in which the potential functioncontains one or more sinks.
 44. The method claim 41 in which thepotential function contains both sources and sinks.
 45. The method ofclaim 41 in which the potential function is that of a toggle switch. 46.The method of claim 40 in which the force field resembles a spring. 47.The method of claim 40 in which the force field resembles a damper. 48.The method of claim 40 wherein the substrate comprises a flat plate. 49.The method of claim 40 wherein the net shear force acts along a singleaxis.